Airworthiness Directives; MD Helicopters Inc. Model MD-900 Helicopters

This amendment supersedes an existing airworthiness directive (AD), applicable to MD Helicopters Inc. Model MD-900 helicopters, that currently requires inspecting the main rotor upper hub (hub) assembly drive plate attachment flange (flange), determining the torque of each flange nut (nut), and if a crack is found, before further flight, replacing the hub assembly. In addition to the current requirements, this action requires visually inspecting the outer surface of the flange at specified intervals, removing the drive plate and visually inspecting the flange for a crack at specified intervals, and replacing any unairworthy hub assembly. This amendment is prompted by reports that cracks starting at the drive plate attachment holes were found in the hub. The actions specified by this AD are intended to detect a crack in the flange and to prevent failure of the hub assembly, loss of drive to the main rotor, and subsequent loss of control of the helicopter.